Interpretive Summary: Soybean seed contains sugars in addition to protein, oil, and other beneficial nutrients. Some of these sugars such as sucrose are desirable because they improve taste and flavor in tofu, soymilk, and natto. Other sugars such as raffinose and stachyose are undesirable because they have detrimental effects on the nutritive value of the meal and are indigestible by human and animals, often causing flatulence or diarrhea in non-ruminants. Therefore, soybean seed with low raffinose and stachyose and high sucrose is desirable. Previous research showed that seed sugars are affected by cultivar, maturity of the cultivar and the environment under which they were grown. In previous studies the effect of cultivar and maturity could not be separated (confounded). To investigate the contribution of each of these variables to individual (sucrose, raffinose, and stachyose) and combined sugars (sucrose+raffinose+stachyose), two sets of isolines were used and a field experiment was conducted in 2004 and 2005. Isolines are soybean lines derived from a common ancestor, therefore they have the same genetic background, but each line differs from the other in one known gene, in this case genes for maturity. One set of isolines was derived from the cultivar Clark, and the other set from Harosoy. The results showed that in 2004 and 2005 the concentration of sucrose, stachyose, and combined sugars decreased as days to maturity increased. Harosoy set showed the same relationship, but only in 2005. The contribution of maturity to the total changes in sugars was greater than the temperature. The effect of genetic background and maturity genes on sugar concentration depended on the type of sugar. This information will help breeders to more efficiently select for seed sugars.

Technical Abstract:
Previous research has shown that variability of total and individual sugars can be attributed to maturity, genotype, environment, and their interactions. However, in those studies maturity and genotype were confounded because genotypes with the same maturity did not have a common genotypic background. The objective of this study was to investigate the effect of maturity and mean maximum daily temperature for the 20 days before maturity on the concentration of specific individual soybean seed sugars (sucrose, raffinose, and stachyose) and combined sugars (sucrose+raffinose+stachyose) in two sets of near-isogenic lines of Clark and Harosoy. The maturity of each line within a set varied, but all had a common genotypic background. Within the Clark isoline set, maturity had a negative linear relationship with the concentration of sucrose (r2=0.83, P<0.0001 in 2004; r2=0.94, P<0.001 in 2005), stachyose (r2=0.51, P<0.0001 in 2004; r2=0.51, P<0.001 in 2005), and combined sugars (r2=0.83, P<0.0001 in 2004; r2=0.91, P<0.001 in 2005). There were no significant relationships between maturity and sugars in the Harosoy set in 2004, but significant negative relationships were found in 2005 with sucrose (r2=0.94, P<0.001), stachyose (r2=0.70, P<0.01), and combined sugars (r2=0.56, P<0.01). In the Clark set, the mean maximum daily temperature had a significant positive relationship with sugars in 2004, but significant negative relationship in 2005. In the Harosoy set, there was a significant negative relationship between maximum mean temperature and sugars only in 2005 (r2=0.66, P==0.0001 for sucrose; r2=0.47, P=0.01 for stachyose, and r2=0.84, P=0.01 for combined sugars). There was no significant relationship between temperature and raffinose in either isoline sets. The contribution of maturity to the total variation in sugars was significantly higher than the mean maximum temperature. Our results indicated that the two isoline sets were different for raffinose and stachyose, but not for sucrose. Hence, the effect of genotypic background and maturity genes on sugar concentration depended on the type of sugar. This information will help breeders to more efficiently select for seed sugars.